Electro-optical devices such as switches and modulators are a key component of current communication systems. On-chip integrated electro-optical devices are becoming an important part of future processors. These devices are required to transfer high density of signals across many processing cores reliably and fast. Another example of potential impact in future processors is the possibility of electro-optical logics. The functionality of many key components of such devices (e.g. modulators, switches etc.) has been demonstrated.
A key part of any integrated optical device is a medium for signal transfer from one location to another across the chip. Usually rectangular waveguides are used for this purpose. Specifically, the signal is transferred using the fundamental mode of these waveguides. Often, these waveguides are designed to accommodate signals with multiple wavelengths at the same time. This means one can transfer/input/output two or more signals in parallel using a single waveguide. Even then, the number of feasible parallel information routes is limited by the spectral response of the waveguide. Should one require more parallel data channels than what fits within a single waveguide, then the simplest option is to add more waveguides. Adding more waveguide clearly occupies more space on the chip. Furthermore, considering the potential coupling effect between these waveguides, one has to position them fairly far apart which in turn adds to the space occupied on the chip.
Another approach for transferring signals truly in parallel across the chip is to use multi-mode waveguides. In this scenario, different signals are modulated in different modes of the multi-mode waveguide and then transferred across the chip. Each mode is still capable of carrying multiple signals with different wavelength. One can think of multi-mode waveguides as two dimensional information transport mediums in which one can not only send different signals with different wavelengths, but also one can send different signals with different modes. A multi-mode waveguide is hereinafter referred to by the term “bus.”
The main challenge in the road map for using buses in integrated optical devices is the problem of multiplexing the signal from a waveguide that may be single-mode or multi-mode to a bus and de/multiplexing the signal from the bus back to the waveguide. A device that can accomplish this goal (i.e. add or drop a mode from/to a bus) is called a mode-division multiplexer (MDM). FIG. 1 shows a schematic view of two such devices 10a, 10b. In FIG. 1, in an example implementation, there is shown a bus 12 (e.g., a multi-mode waveguide) that can carry three modes labeled 1, 2, 3. Each MDM device is made of several components such as mode-selective add/drop units labeled MSADU1, MSADU2 and MSADU3 that interact with the first, second and third mode of the bus, respectively. These components are called mode-selective add/drop units (MSADU). Component SW 15 acts as an on/off switch and is controlled with an electric or another optical signal. In one embodiment, the switch unit 15 decides which mode-selective add/drop unit should be on; and, if so whether it should activate or deactivate a particular MSADU as add or drop (by changing the photons flow direction of the optical signal). The design and operation of component SW 15 is known.
It would be highly desirable to provide a design for and optimization of MSADUs, namely components MSADU1, MSADU2 and MSADU3 as shown in FIG. 1.
Recently, with the growing interest in on-chip integrated optical devices, there are several prior art designs for MSADUs. For example, in a recent proposed approach to MDM based on concept of multi-mode interference, the devices tend to suffer from very large area requirements (on the order of 0.05 mm2), and, have a variable bus structure which may cause problems in matching the required tolerances across different core processors.
Another approach in designing MSADUs is direct adiabatic coupling between the waveguide and bus. These MSADUs have relatively smaller size (on the order of 0.01 mm2) compared to the recent MDM approach supra, however, the size is still too large compared to other typical on-chip integrated electro-optical devices. Although in these designs they have assumed a uniform bus size, they modified the width and more importantly the height of the waveguides for different MSADUs. This causes many problems in usability of their approach in a real multi-core processor.
It would be highly desirable to provide an improved system and method for multiplexing signals from waveguides to an a bus.